1. Field of the Invention
The present invention relates to an optical amplifier for a high power, solid state laser and, more particularly, to an optical amplifier which includes a slab of solid state lasing material, for example, yttrium-aluminum-garnet (YAG) crystal, and one or more diode arrays for exciting the solid state lasing material to a relatively high energy metastable state, wherein the diode arrays are mounted to provide a relatively uniform energy deposition onto the solid state lasing material in a vertical direction in order to reduce non-uniform thermal heating of the crystal to reduce thermal and stress aberrations of the output laser beam.
2. Description of the Prior Art
Solid state lasers are known to include an optical amplifier, for example, a so-called zig-zag amplifier, which, in turn, includes a generally rectangular slab of lasing material, such as yttrium-aluminum-garnet (YAG) crystal. Examples of such solid state lasers are disclosed in U.S. Pat. Nos.: 4,730,324; 4,852,109; and 5,305,345. In such lasers, the atoms in the lasing material are pumped into a relatively high energy metastable state.
Various methods are known for pumping. For example, diode arrays are known to be used in such applications. Lasers which utilize diode arrays for pumping are disclosed in U.S. Pat. Nos.: 4,852,109; 4,949,346; 4,984,246; 5,271,031; 5,305,345; 5,317,585; and 5,351,251. In order to improve performance of the laser, the light energy from the diode arrays must be relatively uniform in the vertical direction of the lasing material in order to reduce thermal and stress aberrations of the resulting laser beam. In such lasers, the light is zig-zagged in a horizontal direction to average out thermal aberrations.
In many known applications, the diode arrays are mounted one on top of another, leaving energy gaps in the lasing material defining unpumped zones 10 in which no light is directed onto the lasing material, as illustrated in FIG. 1. Such a configuration produces a non-uniform deposition of energy in the vertical direction of the lasing material. As discussed above, non-uniform energy deposition results in non-uniform thermal heating of the crystal, resulting in thermal and stress aberrations of the laser beam. More particularly, pumping of the atoms within the lasing material produces considerable heat therewithin in the areas where the atoms of the lasing material are pumped. Since the materials used for many known lasing materials are relatively poor thermal conductors, the non-pumped zones 10 in the lasing material, as illustrated in FIG. 1, cause thermal gradients in the lasing material in the vertical direction.
Various methods are known for minimizing thermal gradients in a lasing material, for example, as disclosed in U.S. Pat. Nos. 4,852,109 and 4,949,346. The '109 patent includes a slab of lasing material formed with a pair of side rails disposed on opposing lateral surfaces of the lasing material. The side rails include a conduit connected to a fluid control system which enables the volumetric flow rate through the conduit within the side rail to be varied which, in turn, allows the temperature of the lateral surfaces to be adjusted. The lateral surfaces of the lasing material are generally parallel to a longitudinal axis of the slab and generally perpendicular to the surfaces subject to energy pumping by diode arrays.
The '346 patent discloses heat sinks bonded to opposing pump faces of the slab of lasing material. In particular, a transparent, thermally conductive heat sink is bonded to the opposing faces of the lasing material, that are subject to the pumping action of the diode arrays.
The configurations disclosed in the '109 and '346 patents attempt to minimize the thermal gradient between the outer surface of the lasing material relative to internal portions. While the above-disclosed methods provide adequate performance in minimizing the effects of the thermal gradient caused between the outer surface of the lasing material and an inner surface, the disclosed methods do not provide compensation for thermal gradients resulting from non-uniform distribution of the energy from the pumping action from the diode arrays, as illustrated in FIG. 1. In order to resolve this problem, U.S. Pat. No. 5,555,254. issued on Sep. 10, 1996 to the same assignee as the assignee of the present invention discloses. A pair of lenses 12 and 14, as illustrated in FIG. 2, disposed between the diode arrays and the crystal. As shown in FIG. 2, the lenses 12 and 14 bend the light rays from the diode arrays to fill in the unpumped zones in the vertical direction of the lasing material. Although such a configuration provides good performance in reducing thermal gradients in the lasing material in the vertical direction, such a configuration is relatively complex and expensive to manufacture. In particular, such lenses are formed by sapphire material, which is relatively expensive.